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Mars not as Earthlike as we thought?

We think of Mars as being Earthlike. Yet Earth formed slowly, while Mars might have formed rapidly to become the runt of the planetary litter.

We think of Mars as a world much like Earth. And yet Earth and Mars might have formed very differently, despite the fact they are the 3rd and 4th worlds in our solar system, and are not vastly different in size. Profound differences in the geologies of Earth and Mars might stem – as two scientists are now suggesting – from Mars’ developing only two to four million years after the birth of our solar system.

In contrast, Earth grew to its full size 50 to 100 million years after the solar system’s birth, via collisions with other small bodies in the solar system.

Mars’s rapid formation might help explain why it is so small – the runt of the planetary litter among inner worlds in our solar system – say the study’s co-authors.

Nicolas Dauphas at University of Chicago and Ali Pourmand at University of Miami made the suggestion of Mars’ rapid formation in a study published in the May 26, 2011 issue of the journal Nature.

Hubble Space Telescope took this picture of Mars on June 26, 2001, when Mars was approximately 68 million kilometers (43 million miles) from Earth – the closest Mars has been to Earth since 1988. Image Credit: NASA and ESA

Relative size of the inner planets of the solar system (from l-r): Mercury, Venus, Earth, and Mars. New research supports the idea that Mars owes its small size to its relatively rapid formation. Image Credit: Lunar and Planetary Institute

Mercury, Venus, Earth, and Mars are sometimes called the terrestrial planets. In other words, the assumption has been that they are like Earth, formed in a similar way and time. But Mars probably should not be considered terrestrial, or Earthlike, said Dauphas, who is an associate professor in geophysical sciences:

Earth was made of embryos like Mars, but Mars is a stranded planetary embryo that never collided with other embryos to make an Earthlike planet.

When he speaks of embryos, he is speaking of bodies that formed within a vast disk of gas and dust – with the infant sun at its center – that existed four-and-a-half billion years ago when our solar system was first forming. Planetary embryos are thought of as bodies big enough to have undergone at least some internal melting, so that the interiors of these bodies have begun a process of forming layers. Planetary embryos are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide.

According to planet formation theory, there was a process of building that went on in the early solar system – smaller planetesimals to larger planetary embryos to actual planets. The idea here is that – while Earth formed slowly from planetary embryos colliding – Mars is itself a solitary planetary embryo that formed very early in the history of our solar system.

The new work provides supporting evidence for this idea, which was first proposed 20 years ago on the basis of planetary growth simulations. The new evidence likely will change the way planetary scientists view Mars. Pourmand, who is an assistant professor in marine geology and geophysics, said:

We thought that there were no embryos in the solar system to study, but when we study Mars, we are studying embryos that eventually made planets like Earth.

There have been large uncertainties in the formation history of Mars because of the unknown composition of its mantle, the rock layer that underlies the crust. Dauphas said:

Now we can shrink those uncertainties to the point where we can do interesting science.

Dauphas and Pourmand were able to refine the age of Mars by using the radioactive decay of the elements hafnium to tungsten in Martian meteorites – that is, meteorites from the planet Mars that have come to Earth. Read more about Martian meteorites here.

To read the details of the process by which Dauphas and Pourmand studied radioactivity decay in Martian meteorites – and arrived at their conclusions – see a May 25, 2011 press release from University of Chicago.

Bottom line: Scientists Nicolas Dauphas of the University of Chicago and Ali Pourmand of the University of Miami have conducted a study on the formation of Mars, published in the May 26, 2011 issue of the journal Nature. Their results show that Mars formed in as little as two to four million years after the birth of the solar system. This is more quickly than Earth formed and would explain the small size of Mars. The study was derived from an examination of radioactive decay in elements found in Martian meteorites.